Various electromagnetic techniques exist to perform surveys of subterranean structures underneath a surface for identifying structures of interest. Examples of structures of interest in the subterranean structure include subsurface resistive bodies, such as oil-bearing reservoirs, gas injection zones, and fresh-water aquifers. One survey technique is the magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter is used to generate electromagnetic signals. With either survey technique, surveying units (or receivers) containing electric and magnetic field sensors are deployed on a surface within an area of interest to make measurements from which a geological survey of the subterranean structure underneath the surface can be derived.
A shortcoming of MT survey techniques is that they are relatively insensitive to thin resistive layers that are typically present in subterranean structures. A thin resistive layer can be a thin (in the vertical direction) reservoir of oil, gas, or fresh water.
Conventional CSEM survey techniques are limited by the so-called air-wave problem, which prevents effective use of the CSEM survey techniques in shallow water applications. In a subsea environment, the air-wave problem is caused by a component of the electromagnetic signal that is generated by the source (the CSEM electromagnetic transmitter), which component can be thought of as following a path upwards from the transmitter to the sea surface, horizontally through the air, and then back down through the sea water to receivers (usually located on a seabed). In deep water applications, the air-wave component does not present a major issue, since the air-wave component is attenuated significantly as the air-wave component passes through the sea water. However, in a shallow water application, the air-wave component is not attenuated as much, so that the air-wave component may dominate the signals that are received by the receiver located at the seabed. In such a scenario, the measurements made by receivers used in a CSEM survey technique would not produce very useful information for the purpose of determining whether a resistive body is located in the subterranean structure.
On land, the air-wave problem is caused by electromagnetic signals propagating from a CSEM electromagnetic transmitter laterally through the air along the air-land interface to receivers. These air-wave electromagnetic signals propagating parallel to the land surface are insensitive to subsurface structures; therefore, such air-wave electromagnetic signals (which are received by the receivers) may interfere with proper detection of resistive objects in a subterranean structure.